The ability to determine the position of mobile telephone or other mobile device connected to radio communications network is becoming a highly desirable feature. The favoured method for providing accurate position information to such devices uses a receiver associated with the device to receive GPS signals from a constellation of GPS satellites. However, because such mobile handsets are often in locations where reception of GPS satellite signals is difficult, eg. inside a car or a building, the availability and accuracy of the GPS positioning provided by the GPS receiver associated with a mobile devices is compromised.
In light of this problem the idea of assisted GPS has emerged. Assisted GPS uses a radio communications network to communicate so called “assistance data” to the mobile station. The assistance data can be used to improve, inter alia, GPS sensitivity, signal acquisition time, accuracy, and battery consumption without requiring additional hardware.
One of the key items of assistance data transmitted to a mobile station equipped with a GPS receiver is a reference time. In CDMA networks the provision of accurate time information is relatively straightforward as the cellular network is synchronised, however this is not the case with GSM or UTMS networks.
U.S. Pat. No. 6,466,164 (Akopian) teaches that in the case where a mobile station cannot receive a GPS signal from a satellite with sufficient signal strength to either demodulate the navigation information or detect bit edge information a suitable estimate of GPS clock time can be obtained from the cellular base station in communication with the mobile station. Akopian teaches that the base station must be provided with a GPS receiver which is able to demodulate the navigation information to thereby determine an accurate estimate of GPS time. This estimate of GPS time is then provided to the mobile station in a time stamped signal. Akopian teaches that a reference time signal derived in the above manner provides a sufficiently accurate estimate of GPS time to allow determination of a pseudorange measurement for the mobile station. However, the draw back of such a solution is that each base station in the cellular network must be provided with a GPS receiver. With very large cellular networks having tens-of-thousands of base stations it will be appreciated that such a solution will be prohibitively expensive to implement in large networks.
Accordingly there is a need for a more cost effective method to provide a sufficiently accurate reference signal to a mobile station in an unsynchronised communications networks.